Re: Testing caps -> NST protection (fwd)

---------- Forwarded message ----------
Date: Tue, 28 Jul 1998 16:18:29 +1200
From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Testing caps -> NST protection (fwd)

Hi Gary,
         Since I said it (and have lived by it.....

> Date: Mon, 27 Jul 98 09:07:53 EDT
> From: Gary Lau  27-Jul-1998 0854 <lau-at-hdecad.ENET.dec-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: Testing caps -> NST protection


> >Finally, I think I read that if the spark gap is close enough to the
> >transformer, secondary RF suppression isn't necessary. Is there agreement
> >on this? 
> >
> >Thanks for your comments.
> >
> >Terry Perdue
> Re. RF suppression, I've heard this suggestion too and I believe this is
> misguided advice.
> Terry Fritz has made some interesting measurements and discoveries about
> the nature of primary gap/tank circuits, in that with each zero-current
> crossing, the gap extinguishes, each time exciting high frequency
> oscillations due to parasitic L-C components.  Measurements with his
> fiber optic voltage probe of Vgap show brief high voltage bursts of 2X
> Vpri at each zero-current crossing.  This is due to C-self of the primary
> inductor, fully charged at a zero-current crossing, resonating with the
> primary inductor.  180 degrees into this VHF oscillation, the voltage
> across C-self reverses and since it is in series with the tank capacitor,
> the two caps in series now present a voltage of  2X Vpri to the gap 
> AND TO THE NST POWER SUPPLY, causing the gap to re-ignite.  This phenominum
> is not influenced by lead length or inductance between the gap and power
> supply (although other oscillations do arise due to this).  A train of 2X
> Vpri (~40KV!!!) voltage bursts applied to an NST is not too healthy for
> it.

If there is that kind of voltage across the main gap, why does it not 
re-ignite? Why should a gap several metres downstream do so? I'm keen 
to know.